While I guess I can see the temptation based on some contemporary promotions, I think one should be quite careful when contemplating designing sewers at flatter slopes (than e.g. per Ten States standards at
. That standard and those minimum requirements appear to be not just based on "lab" tests, but instead the experience of utilities and regulators reflected in multiple editions over at least 55 years comprising and/or influencing the Board in the Ten States area. The slopes you talk about are based on a Manning's "n" of 0.13, and e.g. in the common 8" size achieving a 2 fps velocity that it is hoped will move most normal solids along. Also, while later hinting that a lesser slope might be tolerable in at least some cases,the minimum slope table is preceded with the statement, "However, slopes greater than these may be desirable for construction, to control sewer gases or to maintain self-cleansing velocities at all rates of flow within the design limits", and the table is followed up with the further sort of proviso and perhaps caveat, "Slopes that are slightly less than the recommended minimum slopes may be permitted. Such decreased slopes may be considered where the depth of flow will be 0.3 of the diameter or greater for the design average flow. The operating authority of a sewer system considering decreased slopes shall furnish the appropriate reviewing authority written assurance that any additional sewer maintenance required by reduced slopes will be provided."
You may be interested that quite similar requirements and provisions have also been included in ASCE's "Gravity Sanitary Sewer Design and Construction" (WPCF FD-5) Manuals and reports on Engineering Practice, used in much larger areas and jurisdictions also for many years. The latter reference I know explained, "Generally, Manning's "n" for a given sewer, after some time in service, will approach a constant which is not a function of the pipe material but represents the grit accumulation and slime build-up on the walls. This n will be on the order of 0.013. A coefficient which will yield higher friction losses should be selected for sewers where disturbing influences are known or aniticpated. Because of the empirical nature of each formula, conservative design is prudent." [I suspect there is some testing revealing the basic flow, so to speak, of this passage]
About the only other thing I will say is that I have attended a few Engineering presentations (since first reading these manuals many years ago) indicating that as sewers get much larger, even much higher flow velocities in the realm e.g. of at least 3-4 fps or more etc., may be helpful for large sewers it appears to develop sufficient shear to adequately move most sediment including larger grit and even some rocks etc. that somehow seems to make its way into these larger lines.
